In the exploration and drilling for offshore oil and gas wells, one form of marine structure found to be desirable and effective is the tension leg platform. In this type unit, the working structure is floatably supported by its own buoyancy. However, tension cables applied to the lower end of the platform and fixed to the ocean floor, allow it to be drawn downwardly to a desired working depth.
Under such conditions the structure is less amenable to natural forces such as wind and waves which would otherwise tend to displace and disturb the horizontal orientation of the platform with respect to the ocean floor.
In the drilling of offshore wells, it is necessary to utilize a riser, often referred to as a marine riser, which extends from the well head to the working deck of the floating platform. The riser member is in effect an elongated enclosure which surrounds and protects the drill string as well as pipes which pass from the well upwardly to the platform deck.
Such risers are necessary for normal drilling operations but are susceptible to damage and in many cases to breakage. The latter results from excessive strain applied to the riser as the floating platform vacillates about its working position in response to excessive wind and wave conditions at the water's surface.
Further, the riser is subjected to a considerable stress induced by water currents and the like which pass around the riser, but which are not particularly effective against the platform. In such an instance the normally vertical riser disposition tends to be distorted as the latter is displaced laterally in one or more directions in response to underwater currents.
In the presently disclosed arrangement, the elongated riser of the type contemplated is provided with means directly attached to the tension leg platform such that the disposition and distortion of the riser is readily controlled. The control means includes a flexible connection which extends from the centrally placed riser, outwardly toward the respective tension or hold down cables. Thus, as the riser is subjected to deflecting forces the flexible connection will tend to dampen such movement thus precluding the build-up of excessive local strains.
It is therefore an object of the invention to provide means for supporting a marine riser from a floating tension leg platform positioned at an offshore body of water. A further object is to avoid damage to such a marine riser which would occur if the riser were to be excessively stressed as a result of underwater currents and forces acting thereagainst. Another objective is to provide means for regulating the disposition and alignment of a marine riser through suitable adjustment thereof in a controlled manner from the deck of the floating platform.
Toward achieving the above objectives and toward overcoming the herein mentioned problems, the invention is directed to a bracing system adapted to cooperate with a floating, tension leg type marine structure. The system forms a flexible interconnection between the structure and the marine riser. The riser bracing system includes compensating means for automatically regulating the resisting forces applied to a riser as the latter is deflected from its desired upright disposition.